Methods and apparatus for making a chromatography column

ABSTRACT

Methods for depositing a stationary phase in a tube for chromatography, in particular, as a variable gradient column are disclosed. The methods include providing a tube, loading the tube with a concentration gradient of a stationary phase precursor along its length, and removing the solvent such that the stationary phase precursor is deposited within the tube as a stationary phase to form a chromatography column. The concentration gradient is formed by combining at least a first fluid and a second fluid, with the first fluid containing the stationary phase precursor in a solvent.

TECHNICAL FIELD

The present invention relates generally to methods and apparatus formaking a chromatography column for differential accelerationseparations. More particularly, the invention relates generally tomethods and apparatus for depositing a stationary phase film or coatingon the inner walls of an open tube of any cross-sectional profile with agradually reducing film thickness from one end of the tube to another,or depositing a constant thickness of the stationary phase within a tubethat has a gradually enlarging cross-sectional area.

BACKGROUND

Open tubular columns (OTCs) have been in use in chromatography forseveral decades. They typically consist of an inert tube with astationary phase deposited on the inner walls with a constantcross-sectional area along the length of the tube. The stationary phaseprovides a partitioning phase that permits chemical separation of bandsof analyte molecules carried in the mobile phase passed therethrough.The dimensions of the tube, thickness of the stationary phase, andcomposition of the stationary phase are chosen by the chromatographer toproduce the desired separation.

To form OTCs, two methods are generally used to deposit stationary phaseinside the open tube; dynamic and static stationary phase deposition. Ina dynamic coating process, a plug of solvent, containing dissolvedmaterials that will become the stationary phase, is flowed through thetube such that the dissolved material is left on the inner wall of thetube forming the initial form of the stationary phase film or coating.In some cases, additional treatment steps are implemented, such asheating and drying. These steps seek to condition or chemically alterthe residual stationary phase film or active sites in the tube until thedesired characteristics are provided to the stationary phase. Dynamiccoating is often not preferred because it is difficult to control andtherefore less economic for column production.

In contrast, static coating allows direct, relatively accurate,estimation of the deposited film thickness and can also produce a moreuniform film. In static coating methods, one or more materials that willbecome the stationary phase are suspended or dissolved in a liquid thatis used to fill the tube that will become the OTC. One end of this tubeis sealed off and the other connected to a vacuum source. In someapproaches, a temperature bath may be used to maintain the temperatureof the filled tube. The liquid is then allowed to evaporate, which maybe include the use of a low-pressure vacuum source. During evaporation,the meniscus (air/liquid interface inside the tube) retreats and leavesbehind a residual film of the material(s) that will become thestationary phase. The residual film may then be further chemicallytreated, conditioned, etc to form the final stationary phase.

Variable Geometry Columns (VGC) are disclosed in U.S. Patent PublicationNo. 2009/0139934 to Steinecker et al., herein incorporated by referencein its entirety. That patent application discusses chromatographycolumns that produce differential acceleration based separations byseveral mechanisms. Some proposed mechanisms include increasing thecross-section of the column toward the column outlet, changing thethickness or other composition of stationary phase within the column,and providing a temperature and/or mobile phase velocity gradient alongthe column. Methods and apparatus for efficiently and cost-effectivelymaking VGCs are disclosed herein.

SUMMARY

In one aspect, the invention includes various methods of depositing astationary phase thickness, such as a film or coating, in a tube, inparticular, to form a variable geometry column for differentialacceleration chromatographic separation of analytes including fasterseparation times. The methods include providing a tube, loading the tubewith a concentration gradient of a stationary phase precursor along itslength, and removing the solvent such that the stationary phaseprecursor is deposited within the tube as a stationary phase to form achromatography column. the concentration gradient is formed by combiningat least a first fluid and a second fluid, with the first fluidcontaining the stationary phase precursor in a solvent.

In one embodiment, the tube has a constant cross-sectional area withrespect to its length for achieving differential accelerationchromatographic separation of analytes. In another embodiment, the mayhave an increasing cross-sectional area with respect to its length forachieving differential acceleration chromatographic separation ofanalytes. The stationary phase precursor will ultimately be deposited toform a stationary phase within the tube.

In one embodiment, the chromatography columns made according to theabove methods provide chromatographic resolution of analytes, comparedto a conventional open tubular column, in about one-tenth the separationtime.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of an embodiment of a column of a chromatographicsystem formed from a constant cross-sectional area tube and a graduallydecreasing thickness of stationary phase therein.

FIG. 2 is a side view of an embodiment of a column of a chromatographicsystem formed from a tube having a gradually enlarging cross-sectionalarea and a uniform thickness of stationary phase therein.

FIG. 3 is a side view of an embodiment of a column of a chromatographicsystem formed from a tube having a gradually enlarging cross-sectionalarea and a gradually reducing thickness of the stationary phase.

FIG. 4 is a schematic of an embodiment of a gradient preparation systemfor depositing a stationary phase thickness in an open tube.

FIG. 5 is a flow chart of an embodiment of a method of depositing thestationary phase using an apparatus similar to that illustrated in FIG.3.

FIG. 6 is a schematic of another embodiment of a gradient preparationsystem for depositing a stationary phase thickness in an open tube.

FIG. 7 is a flow chart of an embodiment of a method of depositing astationary phase using an apparatus similar to that illustrated in FIG.5.

FIG. 8 is a photograph of a conventional chromatography column and a VGCprepared using one of the methods and apparatus disclosed herein forsize comparison.

FIG. 9 is side perspective view of a portion of a tube having agenerally rectangular cross-sectional area and having a conductivesurface down the length of the tube for making VGCs.

FIG. 10 contains two gas chromatograms, the top chromatogram being froma conventional open tubular column and the bottom chromatograph beingfrom a VGC prepared using one of the methods and apparatus disclosedherein.

FIG. 11 is a chromatographic resolution versus inlet pressure plot forthe n-heptane/n-octane peak pair separated on a VGC operated in forward(squares) and reverse (circles) flow.

FIG. 12 is a gas chromatogram resulting from a VGC prepared by theelectrochemical method disclosed herein.

FIG. 13 is a side view of a portion of a column similar that of FIG. 3with surface area-increasing member within the tube.

FIG. 14 is a flow chart of an embodiment of a method of depositing astationary phase in a tube to produce a VGC.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments or may be combined in yetother embodiments, further details of which can be seen with referenceto the following drawings and description.

DETAILED DESCRIPTION

Referring to FIG. 1, a side view illustration of a column, generallydesignated 10, of a chromatographic system having a variable stationaryphase film thickness is illustrated. The column 10 includes a tube 11(illustrated as a straight-walled tube, but is not limited thereto)having a mobile phase, designated by arrow 12, flowing from the inlet 16of the tube 11 towards its outlet 18. The stationary phase 14 isdeposited on the inner surface of tube 11 such that the stationary phase14 is thicker at the inlet 16 and thinner at the outlet 18 and thins ina generally linearly manner therebetween. In the arrangement shown inFIG. 1, differential acceleration of the analyte is provided by thevaried thickness of the stationary phase film 14.

The arrangement shown in FIG. 1 may be used for gas chromatography (GC).In this embodiment, the tubular, constant diameter tube 11 is easilyavailable, robust, and compatible with existing gas chromatographs. Forexample, tube 11 can be a polyimide-protected, fused silica column withan undeactivated (native glass) or deactivated inner glass surface. Inone embodiment, the stationary phase film 14 may be polydimethylsiloxanethat may be doped with cross-linking agents that can be deposited in thetube 11 such that the film thickness decreases down the length of thecolumn 10, ideally being as close to zero thickness as possible atoutlet 18. Other suitable stationary phases are discussed below. Thecross-sectional area and length of the tube 11, as well as startingstationary phase thickness, and the rate/function of film tapering, canvary depending upon the chromatography separation that will be performedtherein. However, generally speaking in one case the inner diameter canbe between 50 μm and 500 μm, the film thickness can range from 100-1000nm at the inlet of the column and should be as close to zero at the endof the column as possible, and the length can be about 0.001 to 300meters.

Referring now to FIG. 2, a side view of a column, generally designated20, of a chromatographic system having variable inner diameter/variablecross-sectional area is illustrated. The column 20 includes a tube 21having a mobile phase, designated by arrow 22, flowing from the inlet 26of tube 11 towards its outlet 28. The stationary phase 24 is depositedas a constant thickness on the inner surface of the tube 21. In thearrangement shown in FIG. 2, differential acceleration is provided bythe gradually enlarging cross-sectional area of the column 20 along itslength. The enlarging cross-sectional area may enlarge in a generallylinear manner as illustrated, but is not limited thereto. As shown, thecross-sectional area of column 20 is smaller at the inlet 26 and largerat the outlet 28 while having a uniform thickness of the stationaryphase 24, such that the over all cross-sectional area of the column 20varies steadily and gradually (generally linearly in one case) along thelength of its interior.

In one embodiment, tubes 11 and 21 are not circular in cross-section,but are rectangular in cross-section. In this embodiment, the verticaldimension 29 of FIG. 2 is the width of tube 21. The width is varying,becoming larger in the direction of the outlet 28. The height of tube 21(extending into and out of the page of FIG. 2) may remain constantthroughout the length of tube 21 for ease of fabrication, but may varysimilarly to the width if fabrication capabilities allow. Tube 21 ispreferably coated with a stationary phase 24 of constant thickness. Thisapproach would combine sources of differential acceleration through theexpanding cross-sectional area and decreasing stationary phasethickness, yielding a greater degree of differential acceleration thanwith only a single source of acceleration.

Now referring to FIG. 3, in one embodiment, a chromatography column,generally designated 30, having a variable stationary phase filmthickness is illustrated. The column 30 includes a tube 31 having agradually enlarging cross-sectional area that is narrower at the inlet36 than at the outlet 38. A mobile phase, designated by arrow 32, mayflow from the inlet 36 of the column towards the outlet 38. Thestationary phase 34 is deposited on the inner surface of tube 31 suchthat the stationary phase 34 is thicker at the inlet 36 and thinner atthe outlet 38 and thins in a generally linearly manner therebetween. Inthe arrangement shown in FIG. 3, differential acceleration is providedby the varied geometry provided by the combination of the tube 31 andthe gradient of the stationary phase 34 thickness.

The gas chromatography columns referred to herein as a “column” or“tubular column” may be open tubular columns, but are not limited tocircular cross-sectional areas. The columns may be circular,semi-circular, elliptical, toroidal, oval, square, rectangular,triangular, hexagonal, trapezoidal, parallelogram, or other geometricshapes in cross-sectional area. In another embodiment, the column may bea micro-gas chromatography column made using MEMS technology. Using MEMStechnology, the column may be in the form of an etched wafer ormonolithic chip, for example, as disclosed in U.S. Pat. No. 6,838,640 toWise et al., which is incorporated herein in its entirety. It is wellknown in the art that various MEMS methods will create a variety ofcross-sectional shapes in addition to those listed above.

Disclosed below are methods and apparatus for making VGCs such as thosedescribed above. The methods and apparatus enable the deposition ofcontrolled film thickness gradients in both constant and variablecross-section OTCs as well as the deposition of constant filmthicknesses in variable cross-section OTCs.

One example equation, Equation 1 below, demonstrates one way thedeposited film thickness is governed by the coating parameters whenusing static stationary phase deposition:

$\begin{matrix}{t_{f} = {{r\left( {1 - \sqrt{1 - \frac{c}{\rho}}} \right)} \cong {r\frac{c}{2\rho}}}} & (1)\end{matrix}$where t_(f) is the thickness of the stationary phase film, r is theradius of the un-coated tube (soon to be column), c is the concentration(mass/volume) of stationary phase dissolved in the coating liquid, and ρis the density of the stationary phase in the precipitated or solidform. Compensation factors can be added to the c or ρ terms to accountfor changes to the deposited film during alteration steps as the OTC isconditioned.

Equation 1 makes clear that the thickness of the deposited stationaryphase is directly proportional to the inner radius of the un-coatedtube. Therefore, if one wishes to use traditional static coating methodsto deposit a stationary phase on a tube with increasing cross-sectionalarea as a function of length, the resultant column would have a filmthickness that increases in proportion to the radius of the column; inother words, the stationary phase will not have a constant thickness,and in fact may even have a thickness gradient reversed from thatdesired and thus would not exhibit differential acceleration (i.e. VGCbehavior).

Another example equation, Equation 2 below, is presented that is theanalog of equation 1 for rectangular tubes, that is it gives t_(f) asfollows:

$\begin{matrix}{t_{f} = {\frac{\left( {h + w} \right) - \sqrt{\left( {k + w} \right)^{2} + \frac{4\;{hwc}}{\rho}}}{4} \cong \frac{hwc}{2{\rho\left( {h + w} \right)}}}} & (2)\end{matrix}$Where h is the height of the tube cross-section, w is the width of thetube cross-section, c the concentration of stationary phase precursor inthe static coating solution, and ρ is the density of the depositedstationary phase film. Similarly to equation 1, equation 2 shows thatthe traditional static coating approach will generate a stationary phaseof increasing film thickness with respect to column length if applied toa tube exhibiting the variable cross-section VGC approach.

Equations other than 1 and 2 are possible for other tubingcross-sections; however, because they behave similarly to equations 1and 2, they are not discussed herein.

The methods and apparatus disclosed below can result in a tube with astationary phase therein providing a chromatography column with agradually enlarging cross-sectional area.

Different apparatus for providing such columns are depicted in FIGS. 4and 6, generally designated 100 and 100′ respectively. Each apparatuswill be described in detail below.

Referring now to FIG. 13, in another embodiment, a chromatographycolumn, generally designated as 400, having a variable stationary phasefilm thickness is illustrated. The column 400 includes a tube 431 havinga gradually enlarging cross-sectional area that is narrower at the inlet436 than toward the outlet (not shown). As illustrated, tube 431includes surface area-increasing members 450 therein that havestationary phase precursor deposited thereon as well as on the innersurface of the tube 431 such that there is a greater thickness ofstationary phase precursor in the stationary phase 434 nearest the inletwhich decreases toward the outlet. A mobile phase, designated by arrow432, may flow from the inlet 436 of the column towards the outlet withdifferential acceleration provided by the varied geometry provided bythe combination of the tube 431 and the gradient of the stationary phase434. The surface are-increasing members may be particles or fibers suchas glass fibers packed within the tube 106.

Similarly to a particle-packed column, other structures can be includedin the tube 106 flow path to increase surface area, increase samplecapacity (cross-sectional area of stationary phase), decrease theaverage diffusion distance for analyte molecules to reach the stationaryphase (i.e. increasing column efficiency) or similar. In one embodiment,the tube 106 may be micromachined to have various pathways therethrough.The pathways may provide additional surface area for deposition of thestationary phase precursor for formation of the gradient of thestationary phase discussed above. Micromachined silicon columnscontaining pillars that provide larger sample capacity with betterchromatographic efficiency without increasing pressure restriction haverecently been reported (T. Nakai et al, “Micro-fabricated semi-packedcolumn for gas chromatography by using functionalized parylene as astationary phase, J. Micromech. Microeng., 2009, 19(6), 065032). Theconcentration gradient of the stationary phase precursor to beintroduced into the tube 106 is determined from the desired filmthickness profile and appropriate equation.

The stationary phase precursor may be dissolved, suspended, dispersed,or otherwise carried in an appropriate solvent or other carrier phase.In one embodiment, the stationary phase precursor may be polydimethylsiloxane (PDMS) and the solvent may be n-pentane, dichloromethane,1,2-dichloroethane, benzene, or similar solvent, or a mixture of theseor similar solvents. Other suitable stationary phase precursors include,but are not limited to, polyethylene glycol (PEG), polydiphenyl siloxane(PDPS), polyditrifluoropropyl siloxane. Other suitable solvents include,but are not limited to, volatile solvents such as n-pentane,dichloromethane, 1,2-dichloroethane, benzene, or similar solvent, or amixture of these or similar solvents

Referring now to FIG. 4, apparatus 100 is generally a mechanism forintroducing various concentrations of stationary phase precursor in anappropriately volatile solvent into a tube 106. In one embodiment, thetube 106 may be un-coated. Apparatus 100 includes two or more flowdevices 101, 102 capable of delivering appropriate volumes of fluids122, 124 to the tube 106. The fluids 122, 124 may contain an appropriateconcentration of solute and/or a solvent, wherein the solute istypically the stationary phase precursor. The flow devices 122, 124 areeach connected to the tube 106 by separate tubes 104, 105, respectively,via a junction 103 or via a direct connection to the mouth of the tubeitself. In the embodiment providing a junction 103, junction 103connects the flow devices 101, 102 to tube 106. In one embodiment, tube106 is directly fluidly connected to the junction 103. However, asillustrated in FIG. 3, if desired tubing 107 can be used to join thetube 106 to the junction 103. If tubing 107 is used, proper precautionsshould be taken to account for delays and variations in volume (asdiscussed above in reference to Equations 1 and 2) that tubing 107 mayintroduce. Apparatus 100 may optionally include another component 120such as a waste reservoir, a flow switch or valve, a flow meter, or anyother useful fluidic device, a vacuum pump, or combinations thereofconnected to the outlet 108 of tube 106.

Ultimately, the fluids 122, 124 from the flow devices 101, 102 aredelivered to the tube 106 as a mixture or are simultaneously deliveredto the mouth of the tube where they become mixed. The mixing of thefirst and second fluids 122, 124 is a function of the time and volume tofill the tube 106 with the appropriate stationary phase concentrationgradient profile. With respect to the fluids 122, 124, at least oneshould comprise the stationary phase precursor that will be used toprepare the stationary phase. The stationary phase precursor is carriedin an appropriate solvent such as a volatile solvent. The other fluidmay be pure solvent. Either of the fluids 122, 124 may contain a reagentfor cross-linking of stationary phase, a deactivation agent, otheruseful chemical components, and combinations thereof. For further detailrelating to cross-linking agents, cross-linking and stationary phasedeposition, see Anal. Chem. 2004, 76(9), 2629-2637; and Anal. Chem.2006, 78(8), 2623-2630 which are both incorporated by reference herein.

In one embodiment, the first and second flow devices 101,102 may takethe form of or include syringe pumps and syringes for housing the firstand second fluids 122, 124. In another embodiment, the first and secondflow devices 101, 102 may each take the form of or include ahigh-pressure liquid chromatography (HPLC) pump. Alternately or inaddition to the syring pumps and HPLC pump(s), the flow devices mayinclude a peristaltic pump, a gravity feed device, or other fluiddischarging devices. The flow devices 101, 102 may be controlled by aprocessor programmed to depress the plungers of the syringes orotherwise dispense the fluids at desired flow rates to create theconcentration gradient of the stationary phase precursor within the tube106. The processor may be a computer, microcontroller, data acquisitioncard controlled by a PC and software program, or similar. In anotherembodiment, the first and/or the second flow devices 101, 102 mayinclude a pressure driven reservoir, a shrinking bottle, a HPLC pump, anion chromatography pump, or other similar flow device. It should beappreciated that the first flow device 101 dispenses the first fluid 122and that the second flow device 102 dispenses the second fluid 124, butthere is no requirement that these be housed in/on separate apparatus.It is possible for a single apparatus to includes one or more fluid flowdevices 101, 102 that are operable independent of one another to drivethe dispensing of the first and second fluids 122, 124 at the desiredrates. Alternately, as shown by the optional box 136 enclosing the fluidflow devices 101, 102 and mixing device 103, one apparatus may includesthese components. In an additional embodiment, the multiple flow sourcesare integrated within a single pumping system, such as a gradientelution HPLC pump.

The length and cross-sectional area of the tubes 104, 105 connecting theflow sources 101, 102 to the junction 103 may be kept small relative tothe volume of fluid flowing therethrough to minimize their impact uponerrors in the concentration gradient that will be prepared within thetube 106. The internal volume and fluidic connections of the junction103 are also kept to a minimum for the same reason. Alternatively, ifthe volumes of the tubing 104, 105 and junction 103 are relativelylarge, but of a known value, correction schemes can be employed toaccount for delays and dilution to still deliver a desired concentrationof the stationary phase precursor to the tube. It is possible toback-calculate these volume and delay errors by comparingchromatographic retention factors of columns coated using apparatus 100to columns coated with conventional static coating techniques using thesame first and second fluids 122, 124 in known proportions.

Using an embodiment of the apparatus 100, a method 200 of depositing astationary phase to provide a variable geometry column, generallydepicted in FIG. 5, can be carried out. The method 200 includes a step202 of providing a tube 106 having an inlet and an outlet. Then, step204 includes loading the tube 106 with a concentration of a stationaryphase precursor along its length. The concentration gradient is formedby combining at least a first fluid 122 and a second fluid 124. Thefirst fluid 122 includes the stationary phase precursor in a solvent.Method 200 also includes step 210 where the solvent is removed from thetube to form the chromatography column.

The loading step 204 may include steps 205, 206, and 208. Step 205includes providing a first flow device 101 containing a first fluid 122comprising the stationary phase precursor in a first solvent. The firstflow device 101 being fluidly coupled tube 106. Step 206 includesproviding a second flow device 102 containing a second fluid 124. Thesecond flow device 102 being fluidly coupled to the inlet of the tube106. Step 208 involves simultaneously delivering the first and secondfluids 122, 124 into tube 106 for a period of time to fill the tube.

At step 208 the delivery of the first and second fluids 122, 124includes delivering the first fluid at a first flow rate and deliveringthe second fluid at a second flow rate, which are different flow ratesand which both vary over the time period of filling the tube such that aconcentration gradient of the stationary phase precursor is developedalong the length of the tube. In one embodiment, the first and secondflow rates vary over time while the combined first and second flow ratesremains generally constant. In another embodiment, the first and secondflow rates vary over time while the combined first and second flow ratesvary.

In one embodiment, the first flow rate differs from the second flowrate, where one flow starts at minimum and increases with time to amaximum flow while the other starts at a maximum and decreases with timeto a minimum. The value of the maximum flow depends on the tube'scross-section profile, length, and the desired time required to fill thetube prior to solvent removal. In one embodiment, the value for theminimum flow is zero; however, other minimum flow rates may be desirablein other scenarios (for example, if the target minimum stationary phasethickness is something other than zero). It is possible, but notnecessary, that the maximum and minimum flow rates for the differentflow sources be the same. Depending upon the length of the VGC, it maytake about 3 seconds to about 2 min to fill the tube with the stationaryphase precursor using the methods disclosed herein. Longer time periodsare possible, but not generally necessary with most programmable flowsources such as the syringe pumps in the above embodiment.

In one embodiment, the first fluid has a higher concentration ofstationary phase precursor relative to the second fluid and it isdesired to deposit the stationary phase film thicker at the inlet of thecolumn. Accordingly, the first flow rate will be higher than the secondflow rate at the beginning of the delivering step 208. In order tocreate a concentration gradient of the stationary phase precursor alongthe interior length of the column, as the flow rate of the first fluiddecreases the flow rate of the second fluid will increase proportionallythereby creating a lower concentration of precursor in the mixture ofthe two fluids—yielding a thinner stationary phase film once thedeposition process is completed. During the delivery of the stationaryphase precursor, there may be a point where the first and second flowrate are equal and thereafter the second flow rate will be higher thanthe first flow rate such that even lower concentrations of thestationary phase precursor are created.

In another embodiment, the second flow rate may initially be faster thanthe first flow rate and the process above is simply reversed.

In one embodiment, the first fluid contains at least a stationary phaseprecursor while the second fluid remains pure (i.e. a pure volatilesolvent). In this embodiment, a′ first end of the tube (or both thefirst end and a second end of a tube) is intentionally filled with justthe second fluid (pure volatile solvent) to create a known length ofuncoated column before and/or after the coated region containing thestationary phase. This embodiment is similar to the commonchromatographic practice of using a guard column on either end of ananalytical column; except in this embodiment the “guard column” isintegral to the analytical column which simplifies the configuration forend users and eliminates potential problems with a fluidic junctionbetween the analytical and guard columns. A portion of uncoated firstend of the tube (or of both the uncoated first end and second end) maybe removed prior to use, as is the customary practice in chromatography,especially gas chromatography.

The method 200 may also include the step 210 of removing the solventfrom the tube 106, such that the precursor is separated from the solventand deposited on the walls of the tube. This includes, but is notlimited to, evaporation of a volatile solvent. The solvent removal step210 may be accomplished using known techniques from conventional staticcoating procedures. When the solvent is removed the stationary phaseprecursor is left behind as a stationary phase film or coating on theinterior surface of the tube and thereby forms a chromatography column.The stationary phase will have a thickness t_(f) as a function of columnlength in accordance with Equations 1 and 2 and the ideals ofVGC/differential acceleration. The method 200 may include other stepsknown in conventional static coating procedures for chromatographycolumn preparation such as drying of the stationary phase, conditioningand/or cross linking of the stationary phase, post treating thestationary phase for deactivation or functionalization. In oneembodiment, the outlet 108 of the tube 106, as shown in FIG. 4 isconnected to a valve and vacuum pump 120 such that immediately after thetube 106 is filled with the desired concentration gradient, theevaporation process begins. This approach minimizes evaporation from theend of the tube and therefore reduces risk of forming bubbles at eitherend of the tube 106 during evaporation. The evaporation process may bestopped before solvent is evaporated from the connections to the flowdevices 101, 102 but after evaporation of all liquid in the tube 106 hasbeen completed.

Referring again to FIG. 4, the apparatus 100 may also include a thirdflow device 132 connected by tubing 134 to the end of tube 106 that isopposite the outlet 108 where the solvent removal will occur. Forexample, if evaporation by a pump such as a vacuum pump is occurring,then the third flow device 132 is connected to the end opposite thevacuum pump. The third flow device 132 is used to manipulate remainingliquid during the solvent removal step 210 of process 200. As thesolvent-gas interface (meniscus) retreats, leaving behind stationaryphase precursor, the flow device can be used to pull the liquidbackwards resulting in less stationary phase precursor being depositedthan dictated by the traditional static coating theory (equations 1 and2).

Referring now to FIG. 14 and FIG. 4, in another embodiment, the thirdflow device 132 in combination with the solvent removal may be used tocreate a stationary phase concentration gradient in a tube similar totube 106. In this method, generally designated as 400 in FIG. 14, thefirst step 402 includes providing a tube similar to tube 106 that isfilled with a fluid comprising a stationary phase precursor at onepredetermined concentration. Then, in step 404, one end of the tube 106is connected to a flow device, similar to the third flow device 132 inFIG. 4, and the other end is available for the solvent to exit the tube.The solvent is removed in step 406. The solvent is removed from themeniscus of the fluid composition by operating the flow device toperform step 408. In step 408, as the solvent is being removed todeposit the stationary phase precursor, the third flow device 132 isoperated such that the amount of the stationary phase precursordeposited within the tube changes as the meniscus retreats within thetube to deposit a stationary phase having a concentration gradient alongthe length of the tube. In one embodiment, the third flow device pullsthe fluid, at a changing rate, away from the end of the tube where thesolvent removal is occurring to change the amount of stationary phaseprecursor coating the interior of the tube. This method can be used tocreate VGCs similar to those described above and shown in FIGS. 1-3.

Referring now to FIG. 6, apparatus 100′, for introducing a stationaryphase onto the inner surface of a tube 112, sometimes referred to as anopen tubular column, is illustrated. Apparatus 100′ employs anelectrochemical reaction at the inner surface of the tube 112 to depositthe stationary phase according to the desired film thickness profile(referred to herein as the electroplated stationary phase). In oneembodiment, the tube 112 is a stainless steel chromatographic capillarycolumn. An “iR” drop, that is a drop in electrical potential as a resultof resistance to charge movement through a solution, may be used torestrict an electrochemical reaction as a function of length down thetube 112. In one embodiment, the IR drop is used to electroplate one endof the tube with a thicker layer of stationary phase than the other end.The thickness of the stationary phase may decrease gradually (generallylinearly) from one end of the tube to the other. Tube 112 may have agenerally constant cross-sectional area, or a generally enlargingcross-sectional area (gradually or non-gradually enlarging). Regardlessof which type of tube 112 used, the resulting column with the stationaryphase therein, starting from the inlet of the column, has a generallydecreasing chromatographic phase ratio or generally decreasing retentionfactor from one end thereof to the other. In another embodiment, the iRdrop may be controlled to electroplate the tube with a uniform filmthickness of the stationary phase.

Apparatus 100′ includes an tube 112 having an electrically conductiveinterior surface that is filled with a composition including an aqueoussupporting electrolyte such as NaOH, HCl, NaCl, or the like and a dilutestationary phase precursor such as phenol or the like, and otheradditives such as stabilizers, cross-linkers, or other preparatoryreagents. Within tube 112, the aqueous electrolyte is present in aconcentration sufficient to conduct charge at the rate necessary for theelectrochemical reaction forming the electroplated stationary phase.

Alternatively, a multi-step reaction can be carried out that isinitiated by a reactant precursor that undergoes an electrochemicalreaction at the interior of the tube 112. This includes, but is notlimited, to an acid catalyzed sol-gel process where acid or hydrogenions are generated electrochemically at the interior of the tube 112from a reactant precursor, which then catalyzes a sol-gel formedstationary phase. In another embodiment, the reactant precursor may be asol-gel precursor. Suitable sol-gel precursors that may be applicablehere include water and those disclosed in U.S. Pat. No. 6,783,680 toMalik and WO/2002/072225 to Malik et al. The apparatus 100′ alsoincludes a voltage source 140, such as a potentiostat or the like,connected to the tube 112, in particular electrically connected to theelectrically conductive interior surface of the tube. The voltage source140 may be wired, clipped, or the like to the interior surface of thetube 112. Also connected to tube 112 is a reservoir 115 of an aqueouselectrolyte that is in fluid communication with at least one end of thetube 112. The reservoir 115 may include an aqueous electrolyte 116 suchas KCl at a concentration sufficient to conduct charge at the necessaryrate and optionally any other reagent included in the composition withinthe tube 112. The reservoir 115 may also include additionalelectrochemical devices such as an auxiliary or counter electrode 142and a reference electrode 144 for controlling or monitoring theelectrochemical reactions.

In one embodiment, an electrically insulating union 113 and tube 114connect the end of the tube 112 to the reservoir 115. The electricallyinsulating tube 114 prevents electrochemical plating at the end andouter surface of the tube 112 with stationary phase and insures that allcurrent passed through electrochemical reactions are due to stationaryphase being deposited on the inside of the tube 112. This should alsoallow improved accuracy if using Faradaic charge to estimate the filmthickness profile.

In another embodiment, apparatus 100′ includes an additional reservoir109 connected to the opposite end of the tube 112 in fluid communicationwith the composition therein. The reservoir 109 may contain a reserve ofthe composition filling the tube 112. The reservoir 109 may be connectedto the tube by a connection tube 110 and insulating union 111. Thereservoir 109 may provide a means of flushing the tube 112 with thecomposition filling the tube 112, which includes the stationary phaseprecursor, to ensure that solvent is not lost to evaporation, or toensure that the entire tube 112 is filled uniformly before theelectroplating process begins.

A variety of electrochemical waveforms can be used to drive, a varietyof electrochemical reactions that result in the formation of aninsoluble film or films that can be employed as chromatographicstationary phases. In one embodiment, repeated cyclic voltammograms (CV)are used to react the composition within the tube 112 forming theelectroplated stationary phase. In a composition comprising 5 mM phenolin 25 M NaOH the cyclic voltammograms produce an electrochemicalreaction converting the phenol into a polyphenol stationary phase(-phenyl-O—), which has excellent high temperature stability in bothinert and oxygen-containing carrier gases. Each sequential CV drivesmore electrochemical reactions which results in more depositedpolyphenol stationary phase. However, a maximum thickness may exist dueto passivation of the inner surface (i.e. electrode surface), but inother scenarios surface defects permit sequential layers of polyphenolto be coated on top of each other.

Using an embodiment of the apparatus 100′, a method 300 of depositing astationary phase to provide a variable geometry column, generallydepicted in FIG. 7, can be carried out. The method 300 includes, a step302, providing a tube 112 having an electrically conductive interiorsurface and having a composition within the tube. The tube may besubstantially filled with a composition comprising a stationary phaseprecursor and at least one of an electrolyte or a reactant precursor.Then, step 304 includes electrochemically reacting the reactantprecursor or the stationary phase precursor to form a stationary phasewithin the tube.

Step 304 may include multiple steps 306, 307, and 308. Step 306 includeselectrically connecting the electrically conductive interior surface ofthe tube to a device that provides an electric current or electricpotential. Step 307 includes fluidly connecting one end of the tube 112to a reservoir 115 of a component of the composition within the tube.Step 308 includes activating the device 140 to apply the electriccurrent or electrical potential to the tube 112 to start theelectrochemical reaction step 304. In one embodiment, the component ofthe composition housed within the reservoir 115 is an electrolyte 116.In one embodiment, the electrolyte 116 may be an aqueous electrolyte.The electric current causes an electrochemical reaction that ultimatelydeposits stationary phase on the interior of the tube 112 wherein thecomposition provides an electrical resistance that restricts theelectrochemical reaction of the stationary phase precursor as a functionof the length of the column to form a deposition gradient of thestationary phase therein.

In one embodiment, the method may also include a step of connecting theopposite end of tube 112 to a reservoir 109 of the composition thatfills the tube. The tube 112 may be connected to the reservoir 109 in anempty state such that the method may also include the step of fillingthe tube 112 with the composition from the reservoir 109.

In other embodiments, the natural “iR” drop of the tube may be alteredto achieve a stationary phase with a more constant profile as a functionof tube length. Many methods are clearly possible, one such method, asshown in FIG. 9, would be to create a second conductive surface 402 downthe length of a tube 400 (a small portion of which is shown), such thatit is electrically insulated from the rest of the tube and can serve asthe counter electrode. Arrow 404 designates the direction of movement ofa mobile phase. In effect, this rearranges the “iR” drop such that it isperpendicular to the length of the tube and thereby does not cause achange in the stationary phase profile along the length of the column.

As seen in FIG. 8, a conventional high-speed gas chromatography column A(a 10 mm, 100 μm ID, column) is significantly longer than column B, aVGC, made by the methods disclosed herein. The variable geometry columnB is about twenty times shorter in length than column A. In oneembodiment, the VGC may be about 0.1 inches to about 4000 inches inlength. In another embodiment, the VGC may be about 1 inch to about 40inches in length. In yet another embodiment, the VGC may be about 40inches to 400 inches in length.

Several examples of VGCs are presented below along with experimentaldata demonstrating several aspects of each individual column. For allcases a commercial laboratory-grade GC (7890A, Agilent Technologies,Inc., Santa Clara, Calif., USA) was used for the column evaluation. Theinstrument features a conventional 8-sample automatic liquid samplercapable of injecting 0.02 μL liquid aliquots into the split/splitlessinlet of the instrument. To achieve injections that were as small aspossible to allow operation with low band-broadening columns (i.e.maintain injections that do not significantly add to the extra-columnband broadening), a gas injection was provided consisting of a mixtureof methane gas and n-alkane vapors. By eliminating the evaporation of aliquid sample (500-1000 fold increase in volume), the split injector isable to provide much smaller injection plugs, and therefore allowaccurate evaluation of shorter/faster column configurations. In allcases, the split ratio was adjusted down (i.e. larger injection plug)and the peak width measured to ensure the injection pulse was small withrespect to the on-column band broadening. After evaluation, the data wasprocessed using Grams 32/AI software from ThermoFisher Scientific(Waltham, Mass., USA). Data processing included extraction of the fullwidth at half height (parallel to baseline). Full width at half heightis assumed to be 2.354σ (σ being the standard deviation of the Gaussiandistribution). As is conventional in chromatography, full baseline widthof a chromatographic peak is assumed to be 4σ (95.44% of the integratedarea of the Gaussian distribution).

Example 1 Trial 1

A conventional high-speed gas chromatography column of 50 cm length, 100μm internal diameter, and having a 400 nm thick non-polar stationaryphase of polydimethylsiloxane (Restek, Bellefonte, Pa., USA) wascompared against a VGC prepared using an apparatus similar to thatillustrated in FIG. 3 and described above. The VGC was formed from afused silica capillary column of 50 cm with a 100 μM constant internaldiameter and intermediate polarity deactivation (Restek, Bellefonte,Pa., USA). A non-polar stationary phase of polydimethylsiloxane wasdeposited therein at a variable thickness along the internal diameterthereof with a maximum thickness of 400 nm at the inlet gradually(generally linearly) decreasing to a thickness of 0 nm at the outlet.

Both the VGC and the conventional column were operated under the samecarrier gas flow rate and oven temperatures in a conventional gaschromatograph with hydrogen as the carrier gas and a flame ionizationdetector on the column outlet. The gas chromatograms from Trial 1carried out at 3 psi H₂ carrier gas at 26° C. to separate a diagnosticmixture of C₅ to C₁₀ n-alkanes are shown in FIG. 10.

The VGC separated the C₅ to C₁₀ n-alkanes in less than 14 seconds with an-heptane peak of 760 pA whereas the conventional column required almost4.5 minutes and only had a n-heptane peak of 75 pA. This data shows thatthe VGC, while being a comparable size to the conventional column,produces high quality resolution of the hydrocarbons in one-tenth thetime and with as much as a 10-fold improvement in detectability (pA).

Trial 2

A critical aspect to the proof of concept for the methods disclosedherein is to demonstrate that the stationary phase thickness gradient isindeed created as intended. The simplest experiment for proving this isto evaluate a single gas chromatography column in a GC under firstforward flow conditions and then reverse flow conditions. Then, theforward and reverse results are compared. For a standard chromatographycolumn (i.e., a non-VGC), the forward and reverse chromatograms will bethe same. On the contrary, the forward and reverse chromatograms of aVGC will be significantly different.

To do this, a 50 cm long fused silica tube (Restek, Bellefonte, Pa.,USA), with 100 μM inner diameter and intermediate polarity inner surfacedeactivation was coated with polydimethylsiloxane as described aboveusing apparatus 100. The concentration gradient was prepared such that a400 nm stationary phase film was deposited at the inlet to the column,which tapered down to zero thickness at the outlet of the column.

The column was installed in the GC and a gas phase, analyte samplemixture consisting of methane, n-pentane, n-hexane, n-heptane, n-octane,n-nonane, and n-decane were separated under a series of different inletpressures. Conventionally, chromatographers then calculate the number oftheoretical plates for each component and plot against the averagecolumn linear velocity (calculated from the methane retention time),forming what is known in the art as a Golay plot. Herein, the approachis from a chromatographic resolution perspective, since enhancedseparation between adjacent components is the primary advantage of themethods and resulting columns disclosed, plotted against inlet pressurerather than flow rate. Chromatographic resolution is the difference inretention time between two components divided by the average fullbaseline peak width of the two components (as defined above). Higherflow rates are achieved through higher inlet pressures; however, therelationship between the two is not always a linear function. Thechromatographic resolution versus inlet pressure plot for then-heptane/n-octane peak pair is shown in FIG. 11 for both the forward(square) and reverse (circles) flow directions. At high and low inletpressure, the two flow directions appear to produce nearly identicalseparation performance because band broadening masks the advantages ofdifferential acceleration; however, from 3 to 8 psi, the forward flowdirection produces significantly better chromatographic resolution thanthe reverse direction because of the advantages from differentialacceleration. With a conventional OTC, these forward and reverse flowplots are always identical because the stationary phase profile isconstant along the length of the column.

Example 2

An example chromatogram demonstrating a successful stationary phasecoating using apparatus 100′ of FIG. 6 is shown in FIG. 12. A 2-meterlength of 304 stainless steel tubing with a 0.010″ inner diameter (1/16″ outer diameter) was coated with polyphenol stationary phase usingapparatus 100′ with a coating solution consisting of deionized waterwith a saturated concentration of NaOH (˜25-28 mol/L) and 0.0050 mol/Lphenol. Reference numbers below are referring to FIG. 6. A large areagold electrode severed as a counter electrode 142 and was immersed in a1 mol/L solution of NaCl in the reservoir 116. A standard Ag/AgClreference electrode 144 was also immersed in the reservoir 116. Thepotential of the tube 112 was linearly cycled with time (i.e. cyclicvoltammetry) from 250 mV to 750 mV and back to 250 mV 630 times at ascan rate of 1 mV/sec. Because of the large “iR” drop in the tube, theconventional cyclic voltammogram was not observed (e.g. no oxidationpeak was visible); however, larger anodic currents developed with time,consistent with the sequential growth of polyphenol layers. The columnwas washed with copious deionized water to remove residual ions andmolecules, and then dried under nitrogen gas flow at room temperaturefor 72 hours. The column was then installed in a laboratory GC (thickend at inlet) and a series of chromatographic separations were performedto evaluate the column. The text mixture was a gaseous injection ofmethane, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane,n-undecane, and n-dodecane. Because the column most likely has a thickstationary phase coating, a temperature program was employed to reduceanalysis times to a more convenient length: the oven was held at 30° C.for 45 seconds, then ramped at 20° C./min until reaching 350° C. whereit was held for the remainder of the experiment. The hydrogen carriergas pressure was 2 psi. The injection pulse was proven to be smallrelative to the on-column band broadening by the procedure given above.

The example chromatogram in FIG. 12 shows the fully separated alkanepeaks with insets to the left and right to more clearly show the earlyand late portions of the chromatogram with better graphical resolution.Upon repeated chromatographic analyses, it was found that the retentiontimes for the alkanes in this separation were highly stable, indicatingthe polyphenol film is stable upon repeated temperature cycles up to350° C.

The embodiments of this invention shown in the drawing and describedabove are exemplary of numerous embodiments that may be made within thescope of the appended claims. It is contemplated that numerous otherconfigurations of the devices and methods may be created takingadvantage of the disclosed approach. In short, it is the applicant'sintention that the scope of the patent issuing herefrom will be limitedonly by the scope of the appended claims.

1. A method for depositing a stationary phase in a tube forchromatography, the method comprising: providing a tube; loading thetube with a concentration gradient of a stationary phase precursor alongits length, the concentration gradient being formed by combining atleast a first fluid and a second fluid, the first fluid comprising thestationary phase precursor in a solvent; and removing the solvent suchthat the stationary phase precursor is deposited within the tube as astationary phase to form a chromatography column.
 2. The method of claim1 wherein the chromatography column has an inlet and an outlet, thestationary phase being thicker at the inlet compared to at the outlet ofthe column.
 3. The method of claim 2 wherein the stationary phase variesalong the length of the column as a linear or non-linear function fromthe inlet to the outlet.
 4. The method of claim 2 wherein the tube has agenerally uniform cross-sectional area along its length, and thestationary phase has a generally reducing thickness from one end of thetube to the other end.
 5. The method of claim 2 wherein the tube has agenerally enlarging cross-sectional area along its length; wherein theloading step includes introducing a decreasing concentration of thestationary phase precursor from one end of the tube to the other endthereof; wherein the stationary phase is deposited in the tube as alayer of generally uniform thickness.
 6. The method of claim 2 whereinthe tube has a generally non-uniform cross-sectional area along itslength; wherein the loading step includes introducing a decreasingconcentration of the stationary phase precursor; wherein the stationaryphase is deposited in the tube as a layer of generally non-uniformthickness.
 7. The method of claim 2 wherein the tube includes a narrowercross-sectional area at an inlet thereof relative to the cross sectionalarea at an outlet thereof, and wherein the stationary phase has agenerally reducing thickness from the inlet to the outlet.
 8. The methodof claim 1 wherein the loading step includes simultaneously introducinga first fluid and a second fluid into the tube.
 9. The method of claim 1wherein the loading step includes combining the first and second fluidsin a mixing device before entering the tube.
 10. The method of claim 1wherein the stationary phase precursor is introduced at a higherconcentration at an inlet of the tube relative to the concentration ofthe stationary phase precursor introduced at an outlet of the tube. 11.The method of claim 1 wherein the first fluid is loaded at a first flowrate and the second fluid is loaded at a second flow rate, wherein boththe first and second flow rates vary over time while the combined firstand second flow rates remains generally constant.
 12. The method ofclaim 1 wherein the first fluid is loaded at a first flow rate and thesecond fluid is loaded at a second flow rate, wherein both the first andsecond flow rates vary over time while the combined first and secondflow rates vary.
 13. The method of claim 1 wherein the loading stepincludes substantially filling the tube.
 14. The method of claim 1wherein the loading step includes: providing a first flow device fluidlycoupled to the tube and containing the first fluid; providing a secondflow device fluidly coupled to the tube and containing a second fluid;and simultaneously delivering the first fluid and second fluids, thefirst flow device delivering the first fluid at a first flow rate andthe second flow device delivering the second fluid at a second flowrate; wherein the first flow rate and the second flow rate begin atdifferent flow rates and vary over the loading of the tube to providethe concentration gradient of the stationary phase precursor along thelength thereof.
 15. The method of claim 14 wherein the second fluid is asecond solvent.
 16. The method of claim 15 wherein the second solvent isthe same as the first solvent.
 17. The method of claim 15 wherein thesecond fluid comprises a different concentration of the same stationaryphase precursor in the same volatile solvent as the first fluid.
 18. Themethod of claim 1 wherein the removing step includes evaporating thesolvent.
 19. The method of claim 1 wherein the removing step includescontrolling the temperature of the tube.
 20. The method of claim 14wherein the first and second flow devices include syringe pumps, highpressure liquid chromatography pumps, peristaltic pumps, gravity feeddevices, or a combination thereof.
 21. The method of claim 14 whereinthe loading step includes loading a constant total volume of the firstfluid and the second fluid while simultaneously delivering them.
 22. Themethod of claim 1 wherein the tube includes a surface area-increasingmember within the tube; wherein the stationary phase precursor isdeposited on the surface area-increasing member.